Before you put the gravel into the aquarium, you should clean it so that dirt is removed and does not later swim around in the aquarium. This is also recommended for gravel bought in the pet shop, because this can sometimes still produce dust.
This can be done very easily, for example, by filling the gravel into a bucket (just enough that you can still lift the bucket) and then pouring water on it. Now the gravel is stirred vigorously. The dirt then floats on top of the water. Now pour off the dirty water and fill the bucket with clean water from the tap to the gravel. This is repeated until no more dirt comes out of the gravel when you stir it.
This is quite a strenuous and tedious job. Maybe your parents will help you. But even if you do it alone, you’ll make it! If it is too much for you in one day, you can wash a little bit of gravel every day. But you should then store it in such a way that there are no remains of water in the bucket, because this will rot and stink. Or you fill the already washed gravel in the basin, fill it up with water and turn on the technology. You can then add a little more washed gravel each day until the desired amount is reached.
By the way, some people ask if you have to wash the gravel regularly, for example every time you change the water. But you shouldn’t do that. Once the aquarium is in operation, important beneficial bacteria form in the gravel. If you were to wash it constantly, these die off and a nitrite peak can occur.